To dissipate heat from heat generating semiconductor devices, a number of different approaches and heat dissipating devices are used. Included among these are passive cooling devices, such as heat sinks and integral heat spreaders, and active cooling devices, such as thermoelectric coolers (TECs), fan heat sinks, refrigeration devices, multiphase cooling devices, and others. Yet, in order for any of these to effectively and efficiently cool a heat generating device, they must thermally couple with the heat generating device by some material or method having a relatively low resistance to the flow of thermal energy from the heat generating device to the heat dissipating device.
Commonly, a thermal interface material (TIM) is used between the heat generating device and the heat dissipating device to aid thermal flow. Examples of common TIMs include thermal grease, thermal gaskets, and thin foils of a solder material such as indium. A problem that is common to all of these, however, is void formation between the TIM and either the heat dissipating device or the heat generating device, or within the TIM itself. Voids may be formed by air captured between two flow fronts of a molten material, by volatile by-products of a flux material, or simply by air being trapped between surfaces of materials when a TIM is disposed at a surface of a heat generating or heat dissipating device. Air is a thermal insulator, and so an air-filled void can greatly increase the resistance to thermal flow, reducing the efficiency and effectiveness of an assembly designed to dissipate heat from a heat generating device. Likewise, other trapped gasses, such as flux volatiles, also interfere with efficient thermal flow in such assemblies.
While voids present a problem in most applications, the problem is magnified in thin die, thin TIM (TDTT) applications, because trapped bubbles tend to flatten out and occupy a larger percentage of the total bond line between devices, imposing a greater detrimental effect on the thermal flow from a heat generating thin die chip device to a heat dissipation device. In addition to interfering with thermal flow, voids in TDTT applications can also promote premature package failure due to overheating, heat induced defects (e.g., delamination, cracking).
One approach used to minimize voids is the use of a vacuum oven to remove the flux volatiles during reflow immediately prior to, for example, and indium solder material becoming molten. While effective, vacuum ovens are costly and difficult to implement in high volume manufacturing due to their inherently small capacity. Further, vacuum ovens are ineffective at preventing another source of voiding, that of the inability of a thin preformed solder TIM, as is sometimes used in TDTT applications, to accommodate and fill variable gap widths due to warpage of a thin die chip device.
Voiding in or at the TIM is a major obstacle in TDTT development and manufacturing, particularly as advanced chip device feature sizes decrease in size, and generate increased amounts of heat that must be dissipated.